How to remove a telephone number

boredwiththisissue Hello, this is the official support page for "This phone number is already taken. Sign in with that number or use a different one". As you'll see you can only have one phone number for one Microsoft account. Troubleshoot phone number already taken in Teams - Office Support (microsoft.com)

Adding these on the topic as well.

Adding your phone number to a Microsoft account - Office Support

Change your phone number or email address - Office Support (microsoft.com)

You can use this site to see what accounts are associated with the phone number, or at least get a hint https://account.live.com/username/recover